1. Field of the Invention
The invention relates to a mobile radio communications system, a base station and a controller for such a mobile radio communications system, and also to a method of handing over a call.
2. Description of Related Art
Contemporary cellular mobile radio communications systems are generally formed by a plurality of base stations, in which each base station can be linked to a given number of subscriber stations (=mobile stations) whose locations may change inside a certain geographical area. Such a geographical area will be referenced radio coverage area or radio cell in the following. The radio coverage areas of individual base stations in the mobile radio communications system may then be so large that they cover radio coverage areas of other base stations. As a result, the respective geographical areas of radio cells of different sizes may be covered several times. Depending on the size of the geographical area, the radio cells or radio coverage areas may then be combined to individual hierarchical cell layers. Once a mobile subscriber station moves away from the radio coverage area of a base station with which it is just having a telephone call, care has to be taken that the call to the mobile subscriber station is taken over by another radio cell, i.e. by another base station. The occurrence of such a change of radio cells may be detected, for example, by measurements of the signal field strength, the signal-to-noise ratio, the bit error rate, the distance between base station and mobile subscriber station, and so on. If such a situation is detected, a call handover or handoff may be necessary.
EP 526 436 A1 proposes a method of call handover for a mobile radio communications system with various cell layers. A criterion for a call handover is used the speed of the relevant mobile subscriber station.